1. Field of the Invention
The present invention relates generally to fuel supply systems for providing a compressed natural gas (CNG) fuel to a transit bus or the like, and particularly to the pressure relief side of such systems.
2. Description of the Prior Art
As the search continues for cleaner burning fuels to reduce pollution in the nation's cities, many city transit authorities are converting their bus fleets to run on compressed natural gas, commonly referred to as CNG.
Due to the high pressures at which the CNG must be stored, this presents unique engineering challenges for construction of the fuel systems.
Typically, the fuel on a CNG powered bus is stored in a series of elongated cylindrical tanks. These tanks may either be mounted below the floor of the bus or on top of the roof of the bus.
One example of a roof mounted CNG fuel system for a transit bus is that manufactured by New Flyer. The New Flyer system utilizes a combination of four forward mounted and three rearward mounted Type 4 tank cylinders mounted on top of the bus. The pressure relief system for the New Flyer system ties together two adjacent cylinders. Separate relief lines are connected to outlets on the rear end of each cylinder and are connected to a common relief line. That common relief line carries three separate pressure relief devices spaced along the lengths of the tanks. The pressure relief devices relieve at the occurrence of either an excessive pressure or an excessive temperature in the tanks. The New Flyer system does not provide pressure relief devices directly connected to the ends of the tanks where gas is supplied to the fuel system. Thus, each tank of the New Flyer system is protected by only three relief devices.
Due to the highly flammable nature of the materials involved, it is important to provide as much safety as possible in the safety relief systems.